In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
“Threshold” is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such “capture-verification” algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse energy. This approach, called “automatic capture”, improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, and 2) greatly increasing the device's battery longevity by conserving the battery charge used to generate stimulation pulses.
Commonly implemented techniques for verifying that capture has occurred involve monitoring the internal cardiac electrogram (EGM) signals received on the implanted cardiac electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an “evoked response” may be detected, which is seen as the intracardiac P-wave or R-wave on the EGM that indicates contraction of the respective cardiac tissue. Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by ventricular sensing circuits of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, typically a high-energy back-up stimulation pulse is delivered to the heart within a short period of time in order to prevent asystole. An automatic threshold test is next invoked in order to re-determine the minimum pulse energy required to capture the heart. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter the output level is progressively decremented until capture is lost. The stimulation pulse energy is then set to a level safely above the lowest output level at which capture was attained. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Conventional cardiac stimulation devices include single-chamber or dual-chamber pacemakers or implantable defibrillators. A single-chamber device is used to deliver stimulation to only one heart chamber, typically the right atrium or the right ventricle. A dual-chamber stimulation device is used to stimulate both an atrial and ventricular chamber, for example the right atrium and the right ventricle. It has become apparent in clinical practice that the timing interval between atrial stimulation and ventricular stimulation, known as the AV interval or AV delay, can be important in achieving the desired benefit of dual chamber pacing. Hence, capture verification in each chamber is important in maintaining the desired atrial-ventricular synchrony.
Mounting clinical evidence now supports the evolution of cardiac stimulating devices capable of stimulating both the left and right heart chambers, e.g. the left and right atrium or the left and right ventricle, or even three or all four heart chambers. Therapeutic applications indicated for bi-chamber (left and right heart chamber) stimulation or multi-chamber stimulation include stabilization of arrhythmias or re-synchronization of heart chamber contractions in patients suffering from congestive heart failure. The precise synchronization of the left and right heart chamber depolarizations is expected to be important in achieving the desired hemodynamic or anti-arrhythmic benefit. Thus, verifying capture in each chamber being stimulated would be important in maintaining the desired stimulation benefit.
Sensing an evoked response locally, however, can be difficult because of lead polarization that occurs at the lead-tissue interface whenever a stimulation pulse is delivered. A lead-tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead-tissue interface due to application of an electrical stimulation pulse across the interface. If the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an “afterpotential,” formed at the electrode can corrupt the evoked response signal that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response signal. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn leads to missed heartbeats, a highly undesirable and potentially life-threatening situation. Another problem results from a failure by the pacemaker to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact present, which is also an undesirable situation that will cause the pacemaker to unnecessarily invoke the threshold testing function in a chamber of the heart.
The importance of the problem of lead polarization is evident by the numerous approaches that have been proposed for overcoming this problem. For example, specially designed electrodes with properties that reduce the polarization effect have been proposed.
More stringent signal processing algorithms for analyzing the EGM signal may also be applied in order to detect features that indicate an evoked response is present and distinguish it from a polarization signal. A straight-forward method for analyzing the EGM signal is to set an evoked response sensitivity threshold. If a sensed EGM signal exceeds this evoked response sensitivity threshold within a given timeframe following delivery of the stimulation pulse, capture can be verified. However, the evoked response signal and the polarization signal may be similar in morphology and polarity. Other processing algorithms used to differentiate the evoked response from the polarization signal may include integration of the EGM signal, differentiation of the EGM signal, or template matching of the EGM signal to known depolarization morphologies. However as, processing algorithms become more complicated, additional microprocessing time is required, which is already limited due to the numerous device functions that must be performed, and battery consumption is increased.
Another approach to avoiding the problem of lead polarization is to detect evidence of the actual contraction of the heart chambers by measuring a physiological signal other than the EGM such as blood pressure, blood flow, heart wall motion, or changes in cardiac impedance. The use of additional physiological sensors, however, adds cost, more complicated software and hardware requirements, and increases reliability issues and implant time.
It would therefore be desirable to provide reliable capture verification in a cardiac stimulation device using a method that is relatively straight-forward to implement, and that clearly distinguishes between a locally detected evoked response and the polarization signal without requiring additional sensors or complicated processing algorithms.